Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including light at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. In one such method, often termed laser produced plasma (“LPP”) the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam.
For this process, the plasma is typically produced in a sealed vessel, e.g., vacuum chamber, and monitored using various types of metrology equipment. In addition to generating EUV radiation, these plasma processes also typically generate undesirable by-products in the plasma chamber which can include heat, high energy ions and scattered debris from the plasma formation, e.g., atoms and/or clumps/microdroplets of source material that is not fully ionized in the plasma formation process.
These plasma formation by-products can potentially damage or reduce the operational efficiency of the various plasma chamber optical elements including, but not limited to, collector mirrors including multi-layer mirrors (MLM's) capable of EUV reflection at normal incidence and/or grazing incidence, the surfaces of metrology detectors, windows used to image the plasma formation process, and the laser input window. The heat, high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission or interact with light to produce undesirable heat, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them and/or diffusing into them.
For an LPP system, optics such as the laser beam focusing lens and/or laser beam input window may experience undesirable heat loading due to exposure to the high fluence, e.g. 150-325 W/cm2 from the relatively high powered drive laser, which may be, for example, a pulsed, focused 10-12 kW CO2 drive laser which is synchronized with a droplet generator to sequentially irradiate about 10,000-200,000 tin droplets per second.
In some cases, the laser focusing lens may also function as a laser input window, forming a part of the vacuum chamber, and may be placed under considerable stress due to a pressure differential between the relatively high vacuum in the plasma chamber and the pressure, e.g. atmospheric pressure, outside the plasma chamber. For these windows, deposits and heat can combine to fracture (i.e., crack) the element resulting in a loss of vacuum and requiring a costly repair. To accommodate this stress and prevent fracture, laser input windows have generally been rather thick, and, as a consequence, are subject to thermal lensing. This thermal lensing, in turn, can reduce the ability to properly steer and focus a laser beam to a desired location within the plasma chamber. For example, for use in some LPP EUV light sources, it is contemplated that a lens or mirror be used to accurately focus a high power laser beam to a focal spot sized to irradiate droplets having a diameter between about 10-100 μm.
With the above in mind, Applicants disclose systems and methods for cooling an optic.